python,unittest: don't collect abstract classes

Fix #12275.

changelog/12275.bugfix.rst

@@ -0,0 +1 @@
+Fix collection error upon encountering an :mod:`abstract <abc>` class, including abstract `unittest.TestCase` subclasses.

src/_pytest/python.py

@@ -368,7 +368,11 @@ class PyCollector(PyobjMixin, nodes.Collector, abc.ABC):
             return False
 
     def istestclass(self, obj: object, name: str) -> bool:
-        return self.classnamefilter(name) or self.isnosetest(obj)
+        if not (self.classnamefilter(name) or self.isnosetest(obj)):
+            return False
+        if inspect.isabstract(obj):
+            return False
+        return True
 
     def _matches_prefix_or_glob_option(self, option_name: str, name: str) -> bool:
         """Check if the given name matches the prefix or glob-pattern defined

src/_pytest/unittest.py

@@ -1,6 +1,7 @@
 # mypy: allow-untyped-defs
 """Discover and run std-library "unittest" style tests."""
 
+import inspect
 import sys
 import traceback
 import types
@@ -49,14 +50,19 @@ if TYPE_CHECKING:
 def pytest_pycollect_makeitem(
     collector: Union[Module, Class], name: str, obj: object
 ) -> Optional["UnitTestCase"]:
-    # Has unittest been imported and is obj a subclass of its TestCase?
     try:
+        # Has unittest been imported?
         ut = sys.modules["unittest"]
+        # Is obj a subclass of unittest.TestCase?
         # Type ignored because `ut` is an opaque module.
         if not issubclass(obj, ut.TestCase):  # type: ignore
             return None
     except Exception:
         return None
+    # Is obj a concrete class?
+    # Abstract classes can't be instantiated so no point collecting them.
+    if inspect.isabstract(obj):
+        return None
     # Yes, so let's collect it.
     return UnitTestCase.from_parent(collector, name=name, obj=obj)
 

testing/python/collect.py

@@ -262,6 +262,32 @@ class TestClass:
         result = pytester.runpytest()
         assert result.ret == ExitCode.NO_TESTS_COLLECTED
 
+    def test_abstract_class_is_not_collected(self, pytester: Pytester) -> None:
+        """Regression test for #12275 (non-unittest version)."""
+        pytester.makepyfile(
+            """
+            import abc
+
+            class TestBase(abc.ABC):
+                @abc.abstractmethod
+                def abstract1(self): pass
+
+                @abc.abstractmethod
+                def abstract2(self): pass
+
+                def test_it(self): pass
+
+            class TestPartial(TestBase):
+                def abstract1(self): pass
+
+            class TestConcrete(TestPartial):
+                def abstract2(self): pass
+            """
+        )
+        result = pytester.runpytest()
+        assert result.ret == ExitCode.OK
+        result.assert_outcomes(passed=1)
+
 
 class TestFunction:
     def test_getmodulecollector(self, pytester: Pytester) -> None:

testing/test_unittest.py

@@ -1640,3 +1640,31 @@ def test_raising_unittest_skiptest_during_collection(
     assert skipped == 1
     assert failed == 0
     assert reprec.ret == ExitCode.NO_TESTS_COLLECTED
+
+
+def test_abstract_testcase_is_not_collected(pytester: Pytester) -> None:
+    """Regression test for #12275."""
+    pytester.makepyfile(
+        """
+        import abc
+        import unittest
+
+        class TestBase(unittest.TestCase, abc.ABC):
+            @abc.abstractmethod
+            def abstract1(self): pass
+
+            @abc.abstractmethod
+            def abstract2(self): pass
+
+            def test_it(self): pass
+
+        class TestPartial(TestBase):
+            def abstract1(self): pass
+
+        class TestConcrete(TestPartial):
+            def abstract2(self): pass
+        """
+    )
+    result = pytester.runpytest()
+    assert result.ret == ExitCode.OK
+    result.assert_outcomes(passed=1)